Negative conflict behavior (NCB) in intimate relationships is a problem of daily living that has profound effects on the mental health functioning of relationship partners, the children exposed to NCBs, and society at large. The aim of the proposed study will be to examine, for the first time, the influence of select candidate genes on this novel behavioral phenotype in a sample with well-documented rates of NCB, male combat veterans, and their female partners. NCB will be defined here as a dimension of negative interpersonal behavior ranging in severity from nonverbal expression of hostility to overt acts of violence. It will be measured via a state-of-the-art, multimethod array of self- and partner-reports and through the coding of verbal and nonverbal behavior during a semi-structured conflict discussion between partners in the laboratory. The primary aim of the proposed study will be to examine the direct and indirect effects of psychopathology-linked candidate genes on NCB and the mediating influence of internalizing and externalizing posttraumatic psychopathology on NCB. Analyses will test two primary hypotheses: First, genetic association analyses are expected to show that dopamine and monoamine oxidase (MAOA) polymorphisms exert direct effects on NCB and indirect effects on this outcome mediated by externalizing psychopathology. Second, serotonin and HPA-axis-related polymorphisms are expected to exert direct effects on NCB and indirect effects on this outcome mediated by internalizing psychopathology. Evidence in support of the study hypotheses implicating distinct psychopathological pathways to NCBs could aid in the development and refinement of interventions for NCBs which, to date, have shown only modest effects. In the future, information about genes could be used to inform the selection of pharmacologic agents and guide efforts to match treatments to patients on the basis of their unique genetic architectures.